Electrophotographic material



United States Patent F 3,281,240 ELECTROPHOTOGRAPHIC MATERIAL Paul MariaCassiers, Mortsel-Antwerp, and Robert Joseph Noe, Berchem-Antwerp,Belgium, assignors to Gevaert Photo-Producten N.V., Mortsel, Belgium, aBelgian company No Drawing. Filed Oct. 12, 1961, Ser. No. 144,579 Claimspriority, application Netherlands, Oct. 12, 1960, 256,772 7 Claims. (Cl.96-1) This invention relates to clectrophotographic material and moreespecially to the photoconductive layer of this electrophotographicmaterial.

Electrophotographic materials are known consisting of a support and aphotoconductive layer which contains an inorganic substance as aphotoconductor e.g. selenium, zinc oxide or an organic substance such asanthracene, benzidine or a heterocyclic compound. Further,photoconductive polymers are employed in the manufacture ofphotoconductive layers such as described in the Belgian patentspecifications 588,048, 588,049, 588,050, 599,627 and 604,127.

Most of the known elec-trophotographic materials show the characteristicdisadvantage of having an insufiiciently homogeneous reproduction oflarge black image areas in the original. These large black image areasare only reproduced on their edges by black lines. This effect is calledfringing effect and is described e.g. by Dessauer, Mott and Bogdonoff inPhot. Eng. 6 (1955), pp. 250269. According to these authors the fringingeffect on selenium plates can be eliminated by the use of an auxiliaryor development electrode.

Homogeneously covered image areas may also be obtained e.g. by chargingthe electrophotographic material through an intermittent contact withmacromolecular substances instead of charging the electrophotographicmaterial by an ionisation effect in an air layer, as occurs in thecorona discharge. This method is described in the Belgian patentspecification 568,659.

In another method which aims at the same result, viz. the elimination ofthe fringing elfect, a specific composition of the photoconductive layeris used, containing besides the photoconductive component: zinc oxide,other organic or mineral substances with a grain size and in a weightratio based upon that of the photoconductor. This process is describedin Belgian patent specification 576,367.

It has now been found that for obtaining a visible reproduction of anelectrostatic image by using an electrophot-ographic material comprisinga photoconductive layer containing two different high-molecularcompounds, one of which being discontinuously present in the other,which in its turn is present in the layer in a continuous way and whichconsists of a photoconductive polymer, the product of the dielectricconstant e and the specific resistivity p of said photoconductivepolymer being at least two times greater or smaller than the product ofthe dielectric constant e and the specific resistivity p of thehigh-molecular compound which is discontinuously present in thephotoconductive layer, the quality of the formed image and especiallythe uniformity of the image and the density of the black par-ts aremarkedly improved.

By specific resistivity p of the photoconductive highmolecular compoundis pure understood the specific resistivi-ty measured in the dark, forwhich reason it is also called dark-resistivity.

The determination of the resistivity in the dark of photoconductivepolymers is carried out according to Standard Method of Test forElectrical Resistance of Insulating Materials, A.S.T.M. designations: D257-58 ((41) Volume Resistivity), p. 612, and also D 1371-55 T, p.

3,281,240 Patented Oct. 25, 1966 703, 1958 Book of A.S.T.M. Standards,Part g. The measuring of the specific resistivity of powderymacromolecular substances takes place according to the method describedin Chemie-Ingenieur-Technik, 31, 1959, pp. 43- 45. The measuring of thedielectric constant takes place by means of the same measuring cell asfor the specific resistivity, but with the help of a Schering-bridge.Although the measurings on powdery substances give only approximativevalues, they suflice for the purpose of this invention, since only theratio of the values and not the absolute values are of importance. Thereproducibility of these measurings improves by vibrating the abovecited measuring cell until an optimal volume concentration of the powderis reached. In respect of the examples below, the measuring results onthe photoconductive polymers and the macromolecular compounds dispersedtherein are now given:

e p in eXp ohm.cm.

(1) Hostalit CAM (registered trademark) 3. 43 2. 2X10 7. 4X10 (2) Starch7.5 2.6X10 2x10 (3) Polyvinylcarbazole 3.05 5.8)(10 1.8 10 (4) Methylenestarch 9. 15 6.2)(10 5.7)(10 See preparation 1Methylene urea starch.

For the application of the process of this invention are considered e.g.the photoconductive polymers described in the Belgian patentspecifications 588,048, 588,049, 58 8,050, 570,790, 599,627 and 604,126,and also the photoconductive material from this last mentionedspecification.

The following macromolecular additives are considered.

CELLULOSE DERIVATIVES acetate cinnamate, and cellulose acetatephthalate.

POLYSACCHARIDES AND PROTEINS Polyethylene, polytetrafiuoroethylene,polyamides of the adipic acid-hexamethylene diamine type, polyamides ofthe caprolactam type, rubber, phenol-formaldehyde resins, andurea-formaldehyde resins.

VINYL POLYMERS The chlorine containing polymers and copolymers cited inBelgian patent specification 604,126, and more especially the followingpolymers and copolymers of vinyl chloride: Geon Resin 100, Geon Resin101, Geon Resin 202, Geon Resin 203, Geon =Resin 204, Geon Resin 205 andGeon Resin 404 (trade-names of British Geon Ltd., London).

Aluminum polysilicates e.g. porcelain powder and kaolin.

The following are preparations of some of the above cited products.

1. Preparation of a reaction product of formaldehyde, ureum, maleicanhydride and starch called methylene urea starch.

110 g. of undried starch, which corresponds to 100 g. of dry product,are Suspended in a solution of 11.5 g. of urea and 110 cm. of water.After staying for 1 /2 h., 400 cm. of acetone, 33.5 cm. of 40% aqueousformaldehyde, 1 g. of maleic anhydride and 12 cm. of water are addedthereto. This reaction mixture is stirred for 2 h. at 40 C., whereuponthe temperature is raised to 60 C. The mixture is stirred at thistemperature for 4 h. Then the reaction mixture is cooled and the solidproduct is sucked off. The filter cake is washed with water until thewashings are free of formaldehyde. The water in the filter cake iseliminated by acetone. The obtained product is dried under ventilation.

2. Preparation of starch-N-phenylcarbamate 55 g. of undried rice starchare dehydrated with benzene by an azeotropic distillation during whichcare is taken that the volume is kept constant by regular addition ofwater-free benzene.

After drying by distillation, 20 cm. of phenyl isocyanate are added.After refluxing for 6 h., the formed solid product is sucked off. Thisproduct is Washed with ethanol, sucked off again and dried.

Polymeric compounds which are used as additives for the elimination ofthe fringing effect, areinsoluble or are made insoluble in the usualsolvents. This insol'ubiliz'ation can take place during thepolycondensation if a synthetic product is concerned e.g. bycross-linking or hardening, or by an after-treatment after the formationof the polymer, e.g. photopolymerization, heat treatment, chemicalhardening or vulcanization.

The size of the particles of the added macromolecular compoundspreferably varies between 1 and a. The particles can occur in the formof grains, fibres or scales, without excluding, however, other particleforms.

In practice, the polymeric photoconductive substance used While Wet isground with other additives as is described more specificallyhereinafter in a solvent for the photoconductive components, until therequired dispersion and grain size of the macromolecular insolublecompounds are obtained.

The solvent is chosen in such a way, that the particles to be dispersedof the high-molecular compound remain in their undissolved state whereasthe photoconductive polymer can dissolve.

The obtained dispersion is uniformly spread over the surface of asupport e.g. by centrifugation, spraying, brushing or coating, whereuponthe formed layer is dried in such a way, that a uniform photoconductivelayer is obtained.

Preferably the amount of macromolecular dispersed particles amounts to 5to 50% based on the weight of the photoconductor.

The thickness of the photoconductive layers is not critical, but is opento choice within a wider range according to requirements. Good resultsare obtained with photoconductive layers having a thickness of between 1and 20 but preferably of between 3 and 10;. Layers which are too thinhave an insuificiently insulating power and layers which are too thickrequire extensive exposure times.

It is well understood that the invention is not limited by the use ofdispersions of the above mentioned macromolecular compounds in solutionsof previously polymerized compounds, but that e.g. dispersions inmixtures of monomeric and polymeric substances can be applied onto thesurface of the support to be coated, and that these may be polymerizedin situ, condensed or cross-linked according to one of the known methodsin polymer chemistry.

The photoconductiv'e layers manufactured according to this invention,besides one or more photoconductive polymeric substances, may stillcontain one or more photoconductive monomeric compounds.

As monomeric photo'conductors are considered e.g. the photoconductivecompounds mentioned in the U.S. patent specifications 2,599,542(anthracine) and 2,663,636 (anthraquinone, acenaphthene, fluoranthene,naphthalene, chrysoquinone, pyrogallic acid and microcrystalline Waxes),the French patent specification 1,176,381 and the published Germanpatent specification 1,060,712, the Belgian patent specifications558,078, 558,630, 562,336, 562,426, 563,045 and 570,790, and theAustrian patent specification 205,516 (p-diphenyl benzene, benzanthroneand aromatic nitriles).

To the composition of the photoconductive layer, which contains one ormore polymeric photoconductors and one or more monomericphotocondu-ctors, a binding agent may be added for obtaining amechanically suificiently strong subbing layer, such as described in theBelgian patent specification 585,555. If desired, plasticizers may beadded which are also described in that same patent specification.

In the photoconductive layers according to this invention there canfurther be present compounds which occasionally possess photoconductiveproperties and which cause an increase of the general sensitivity and/or of the sensitivity for electromagnetic rays from a defined part ofthe spectrum. Such compounds are described for instance in Belgiumpatent specification 588,050, p. 12-22, classes A to T inclusive. Thesecompounds are pref- 'er'ably employed in amounts ranging from 0.1 to 5%based upon the weight of the used polymeric photo'conductive substances.

Other additives, well known in the art of coating, which may be used,include agents controlling ageing, oxidation, gloss, thermal stability,electric conductivity, mechanical resistance, viscosity or otherphysical properties. In selecting such additives, preference is given tothose substances which do not markedly decrease the dark-resistivity ofthe photoconductive layer.

Suitable supports for the electrophoto'graphic layer are described inBelgian patent specification 588,050.

For charging and exposing of the electrophotographic material,developing and fixing of the image, this same patent specification andthe article of Dessauer, Mott and Bo'gdonoff, Phot. Eng. 6 (1955), pp.250-269, may be cited.

As a matter of fact this new invention should by no means be limited toone or other special embodiment, in regard to the use of the newelectrophotographic materials, the method of charging, the exposuretechnique, the transfer (if any), the developing method, and the fixingmethod as well as the materials used in these steps can be chosenaccording to requirements.

Electrophotographic materials according to the present invention can beapplied in reproducing techniques using different kinds of radiations,not only electromagnetic radiations as hereinbefore referred to but alsonuclear radiations. For this reason, it should be pointed out thatalthough materials according to the invention are mainly intended forapplication in processes involving an exposure, the termelectrophotograp-hy wherever appearing in the description and the claimsis used broadly, and includes both xerogra-phy and xeroradiography.

The following examples illustrate the present invention withoutlimiting, however, the scope thereof.

EXAMPLE 1 A baryta-coated paper of 90 g./sq. m. bearing g. of baryta persq. m., is coated with a layer from a 12% solution of Hostalit CAM(trademark of Farbwerke Hoechst A.G., Frankfurt am main-Hochst for a terpolymer of vinyl chloride, vinyl acetate and maleic anhydride) inacetone. The thickness of the dried layer is 5 This support is thencoated with a layer from the following composition:

10% solution of Hostalit CAM (trademark) in acetone cm 50 10% solutionof leuco malachite green in methylene chloride -cm 50 Starch (grain sizeof about 1 g 0.4

After completely drying, this material is negatively charged with acorona device. This material is exposed for 20 sec. through adiapositlve with a 100 Watt lamp placed at a distance of 10 cm. Thelatent electrophotographic image is then developed with a mixtureconsisting of 5 g. of Graph-O-Fax Toner No. 3 (trade-name of Philip A.Hunt Company, Palisades Park, N.J., for a xerographic developingdyestufi?) as a toner, and 100 g. of iron powder as a carrier.

By fixing according to one of the known methods, a strong and sharpprint having especially well covered black areas, is obtained.

EXAMPLE 2 A mixture of the following composition is ground for 12 h. ina ball-mill:

Polyvinylcarbazole (prepared according to the method described inBelgian patent specification 589,995) cm 100 l-chloroanthraquinone g 0.1Methylene ureum starch (prepared according to preparation 1) g lMethylene chloride cm 100 A mixture of the following composition isground for 12 h. in a ball-mill:

Copolymer of vinylcarbazole and ethyl acrylate (prepared according tothe method described in Belgian patent specification 589,995) g 7.5Basic blue 3 (Cl. 51,005) mg 4 Starch-N-phenyl carbamate (preparedaccording to preparation 2) g 1 Methylene chloride cm 100 Abaryta-coated paper of 90 g. per sq. m. bearing 10 g. of baryta per sq.m., is coated with a layer from this composition.

After drying the thickness of this photoconductive layer amounts to 10The material is then charged with a corona device and exposed in anenlarger with a lamp of 75 watts. A linear enlargement of 10 times of adia positive requires an exposure of 2 min. The obtained material isdeveloped with a tribo-electric powder as described in Example 1. Avigorous image is obtained having a clear background and well coveredblack parts.

We claim:

1. In a method of reproducing an original by the creation of a patternof electrostatically charged and uncharged areas according to saidoriginal on a supported normally insulating layer becoming conductiveupon exposure to electromagnetic radiation, which method includes thesteps of electrostatically charging said layer and exposing said layerto an image of said radiation according to said original to create saidpattern and developing said charged areas of said pattern withfinely-divided electrostatically attractable material attracted by thecharges therein but not attracted to the uncharged areas thereof, theimprovement wherein said supported layer is comprised essentially of aninsulating organic polymer becoming conductive upon said exposure andhaving at least substantially uniformly distributed therethroughdiscrete finely divided particles of a polymeric substance lessconductive upon exposure to said radiation than said insulating organicpolymer, the arithmetic product of the dielectric constant and thespecific electric resistance of said insulating organic polymer measuredin the dark differing by a factor of at least two from that of saidpolymeric substance, said polymeric substance being present in saidlayer in the amount of about 5-50% by weight of said insulating organicpolymer.

2. A method according to claim 1 wherein electrophotographic material issubjected to a substantially uniform electrical field to produceelectrostatic charge completely thereover and is thereafter exposed tosaid electromagnetic radiation image to create said pattern ofelectrostatic charges.

3. An electrophotographic material for use in the method of claim 1comprising a supported layer comprised essentially of a continuous phaseof an insulating organic polymer becoming conductive upon exposure toelectromagnetic radiation, said polymer phase having at leastsubstantially uniformly distributed therethrough a separate dispersedphase of discrete finely divided particles of a polymeric substance lessconductive upon exposure to said radiation than said insulating organicpolymer, the arithmetic product of the dielectric constant and thespecific electric resistance of said insulating polymer measured in thedark differing by a factor of at least two from that of said polymericsubstance, the amount of said polymeric substance in said layer beingabout 550% by weight of said insulating organic polymer.

4. The material of claim 3 wherein the finely divided particles of saidpolymeric substance are of a size of about 1-5 microns.

5. The material of claim 3 wherein said layer contains in addition tosaid organic polymer and said polymeric substance, at least onemonomeric compound becoming conductive upon said radiation.

6. The material of claim 3 wherein said layer containsin addition tosaid organic polymer and said polymeric substance, an agent increasingthe radiation sensitivity of said polymer.

7. The material of claim 3 wherein said layer has a thickness of about1-20 microns.

References Cited by the Examiner UNITED STATES PATENTS 3,011,918 12/1961Silvernail et al 96-1 3,037,861 6/1962 Hoegl 961 3,159,483 12/ 1964Behmenberg et al. 96-1 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

1. IN A METHOD OF REPRODUCING AN ORIGINAL BY THE CREATION OF A PATTERNOF ELECTROSTATICALLY CHARGED AND UNCHARGED AREAS ACCORDING TO SAIDORIGINAL ON A SUPPORTED NORMALLY INSULATING LAYER BECOMING CONDUCTIVEUPON EXPOSURE TO ELECTROMAGNETIC RADIATION, WHICH METHOD INCLUDES THESTEPS OF ELECTROSTATICALLY CHARGING SAID LAYER AND EXPOSING SAID LAYERTO AN IMAGE OF SAID RADIATION ACCORDING TO SAID ORIGINAL TO CREATE SAIDPATTERN AND DEVELOPING SAID CHARGED AREAS OF SAID PATTERN WITHFINELY-DIVIDED ELECTROSTATICALLY ATTRACTABLE MATERIAL ATTRACTED BY THECHARGES THEREIN BUT NOT ATTRACTED TO THE UNCHARGED AREAS THEREOF, THEIMPROVEMENT WHEREIN SAID SUPPORTED LAYER IS COMPRISED ESSENTIALLY OF ANINSULATING ORGANIC POLYMER BECOMING CONDUCTIVE UPON SAID EXPOSURE ANDHAVING AT LEAST SUBSTANTIALLY UNIFORMLY DISTRIBUTED THERETHROUGHDISCRETE FINELY DIVIDED PARTICLES OF A POLYMERIC SUBSTANCE LESSCONDUCTIVE UPON EXPOSURE TO SAID RADIATION THAN SAID INSULATING ORGANICPOLYMER, THE ARITHMETIC PRODUCT OF THE DIELECTRIC CONSTAND AND THESPECIFIC ELECTRIC RESISTANCE OF SAID INSULATING ORGANIC POLYMER MEASUREDIN THE DARK DIFFERING BY A FACTOR OF AT LEAST TWO FROM THAT OF SAIDPOLYMERIC SUBSTANCE, SAID POLYMERIC SUBSTANCE BEING PRESENT IN SAIDLAYER IN THE AMOUNT OF ABOUT 5-50% BY WEIGHT OF SAID INSULATING ORGANICPOLYMER.